The relationship of the physiology and dynamics of myometrial activity to the onset of labor and the distribution of uteroplacental circulation will be evaluated in monkeys. Specific attention will be directed towards the relationships of steroid hormones, prostaglandins and adrenergic receptors in modulation of the demonstrated biorhythms of uterine spontaneous activity and hemodynamics and the role of these agents on the distribution of blood flow to placental and nonplacental uterine tissues. Chronically implanted sensors will monitor intrauterine pressure, arterial and venous pressure, and blood flow in unanesthetized animals conditioned to restraining chairs. Radioisotopes and other indicators will be used to analyze various aspects of placental and nonplacental uterine blood flow. Electronic integration and computer analysis of measured parameters will facilitate evaluation of the studies in an effort to identify and assess some of the factors that control uterine activity and the mechanisms of fetal and maternal uterine homeostasis. A primate breeding colony will be maintained.